Input voltage sense circuit in a line powered network element

ABSTRACT

Methods and systems of monitoring voltage input to network elements are provided. One method includes receiving an input voltage for a line-powered network element, applying the input voltage across a primary winding of a flyback power converter during an on-time of a primary switch, and transferring the input voltage from the primary winding to a secondary winding of the flyback power converter during an off-time of the primary switch. The method further includes sensing the voltage of the secondary winding during the on-time of the primary switch; and drawing minimal current from the flyback power converter.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of co-pending application Ser. No.10/449,496, filed on May 30, 2003 and entitled INPUT VOLTAGE SENSECIRCUIT IN A LINE POWERED NETWORK ELEMENT.

This application is related to co-pending application Ser. No.10/134,323, filed on Apr. 29, 2002 and entitled MANAGING POWER IN A LINEPOWERED NETWORK ELEMENT (the '323 Application). The '323 Application isincorporated herein by reference.

This application is also related to the following applications:

Application Ser. No. 10/449,910, filed on May 30, 2003 and entitled“FUNCTION FOR CONTROLLING LINE POWERING IN A NETWORK,”

Application Ser. No. 10/449,259, filed on May 30, 2003 and entitled“LINE POWERED NETWORK ELEMENT,”

Application Ser. No. 10/449,682, filed on May 30, 2003 and entitled“ELEMENT MANAGEMENT SYSTEM FOR MANAGING LINE-POWERED NETWORK ELEMENTS,”

Application Ser. No. 10/449,917, filed on May 30, 2003 and entitled“CURRENT SENSE CIRCUIT IN A LINE POWERED NETWORK ELEMENT,”

Application Ser. No. 10/448,884, filed on May 30, 2003 and entitled“LIGHTNING PROTECTION FOR A NETWORK ELEMENT,”

Application Ser. No. 10/449,546, filed on May 30, 2003 and entitled“SPLITTER,” and

Application Ser. No. 10/449,547, filed on May 30, 2003 and entitled“POWER RAMP-UP IN A LINE-POWERED NETWORK ELEMENT SYSTEM,” all of whichare incorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to the field oftelecommunications, and, in particular, to input voltage sensing in aline powered network.

BACKGROUND

Telecommunications networks transport signals between user equipment atdiverse locations. A telecommunications network includes a number ofcomponents. For example, a telecommunications network typically includesa number of switching elements that provide selective routing of signalsbetween network elements. Additionally, telecommunications networksinclude communication media, e.g., twisted pair, fiber optic cable,coaxial cable or the like that transport the signals between switches.Further, some telecommunications networks include access networks.

For purposes of this specification, the term access network means aportion of a telecommunication network, e.g., the public switchedtelephone network (PSTN), that allows subscriber equipment or devices toconnect to a core network. For example, an access network is the cableplant and equipment normally located in a central office or outsideplant cabinets that directly provides service interface to subscribersin a service area. The access network provides the interface between thesubscriber service end points and the communication network thatprovides the given service. An access network typically includes anumber of network elements. A network element is a facility or theequipment in the access network that provides the service interfaces forthe provisioned telecommunication services. A network element may be astand-alone device or may be distributed among a number of devices.

There are a number of conventional forms for access networks. Forexample, the digital loop carrier is an early form of access network.The conventional digital loop carrier transported signals to and fromsubscriber equipment using two network elements. At the core networkside, a central office terminal is provided. The central office terminalis connected to the remote terminal over a high-speed digital link,e.g., a number of T1 lines or other appropriate high-speed digitaltransport medium. The remote terminal of the digital loop carriertypically connects to the subscriber over a conventional twisted pairdrop.

The remote terminal of a digital loop carrier is often deployed deep inthe customer service area. The remote terminal typically has line cardsand other electronic circuits that need power to operate properly. Insome applications, the remote terminal is powered locally. In somenetworks, the remote terminal is fed power over a line from the centraloffice. This is referred to as line feeding or line powering and can beaccomplished through use of an AC or a DC source. Thus, if local powerfails, the remote terminal still functions because it is typicallypowered over the line using a battery-backed power source. This allowsthe remote terminal to offer critical functions like lifeline plainold-fashioned telephone service (POTS) even during a power outage.

Over time, the variety of services offered over telecommunicationsnetworks has changed. Originally, the telecommunications networks weredesigned to carry narrowband, voice traffic. More recently, the networkshave been modified to offer broadband services. These broadband servicesinclude services such as digital subscriber line (DSL) services. As timegoes on, other broadband services will also be supported. These newservices often come with increased power requirements.

Line-powered network elements in access networks rely on the centraloffice for continuous power. As the distance between the central officeand a network element increases, the amount of power required to providea constant voltage at the network element increases. In some instances afaulty channel card, a short on a channel card or even improperinstallation of a channel card causes an increase in the current draw atthe line powered network element. The current increases and the voltagereceived at the line powered network element (CPE, RT) decreases. Whenthe input voltage at the network element begins to fall an indicator isneeded to prohibit a power source shut down.

Input voltage may be too low, and current draw may be too high for manyreasons. If the span used for powering the network elements is too highin resistance, the voltage drop on the span line will be large. In someinstances, this would occur with an improper installation of equipment.In this situation, the voltage at the network element sink is lower.Since the network element sink will consume a fixed amount of power, itmust consume more current to operate at a lower voltage. A criticalpoint may be reached where the voltage drop on the span equals the inputvoltage at the network element sink, and is one half of the networkelement source output voltage and power. If the current increases beyondthis point the network element sink power supply will drop out and ceaseto operate. At this point the power system will go through a re-bootprocess. As this is lengthy and will cause a service outage, thissituation must be avoided.

Therefore, there is a need in the art for detecting line input voltagefor line powered network elements and to provide an indicator.

SUMMARY

A method of monitoring input voltage for a network element is provided.The method includes receiving an input voltage for a line-powerednetwork element, applying the input voltage across a primary winding ofa flyback power converter during an on-time of a primary switch, andtransferring the input voltage from the primary winding to a secondarywinding of the flyback power converter during an off-time of the primaryswitch. The method further includes sensing the voltage of the secondarywinding during the on-time of the primary switch and drawing minimalcurrent from the flyback power converter.

An apparatus is provided. The apparatus includes a flyback powerconverter and a monitoring circuit coupled to a secondary side of theflyback power converter. The monitoring circuit selectively senses aninput voltage to the flyback power converter from a reflected voltage ofthe input voltage, compares the input voltage to a reference voltage andprovides an indication when the input voltage differs from the referencevoltage by a defined amount. The monitoring circuit is adapted to drawminimal current from the flyback power converter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of one embodiment of an monitoring circuit coupledto a flyback power converter according to the teachings of the presentinvention.

FIG. 2 a is an illustration of a waveform of the primary currents on aflyback power converter according to the teachings of the presentinvention.

FIG. 2 b is an illustration of a waveform of the secondary currents ofone embodiment of a flyback power converter according to the teachingsof the present invention.

FIG. 2 c is an illustration of a waveform of the transformer primaryvoltage of one embodiment of a flyback power converter according to theteachings of the present invention.

FIG. 3 is one embodiment of a monitoring circuit shown generally at 310,according to the teachings of the present invention.

FIG. 4 is one embodiment of a voltage sensing circuit, shown generallyat 420, according to the teachings of the present invention.

FIG. 5 is another embodiment of a monitoring circuit shown generally at510, according to the teachings of the present invention.

FIG. 6 is a block diagram of one embodiment of a network that includesat least one line-powered network element according to the teachings ofthe present invention.

FIG. 7 is one embodiment of a wireless network according to theteachings of the present invention.

FIG. 8 is a block diagram of one embodiment of a central officeaccording to the teachings of the present invention.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings that form a part hereof, and in which is shown byway of illustration specific illustrative embodiments in which theinvention may be practiced. These embodiments are described insufficient detail to enable those skilled in the art to practice theinvention, and it is to be understood that other embodiments may beutilized and that logical, mechanical and electrical changes may be madewithout departing from the spirit and scope of the present invention.The following detailed description is, therefore, not to be taken in alimiting sense.

Embodiments of the present invention address problems with providingpower over communication lines to line powered network elements in anaccess network. Particularly, embodiments of the present inventionprovide a monitoring circuit for line powered network elements thatmeasures the input voltage at the line powered network element.

Embodiments of the present invention provide methods and systems formonitoring the input voltage of line powered network elements. Anincrease in current draw can be detected by a decrease in the inputvoltage received at the line powered network element. Line powerednetwork elements such as customer premises equipment rely on powerprovided by a central office, remote terminal, or other network element.In one embodiment, a voltage sensing or monitoring circuit is employedto monitor a representation of the input voltage to the network elementas an indicator of the line power headroom of the network element. Themonitoring circuit monitors the input voltage and draws a small amountof current. In some applications there is a need to reduce the powerconsumed by line cards in the network element. If there is a faultychannel card that draws more power than the card is designed for, thereis a need for a way to detect it and flag it. In other embodiments theobjective is to lower power usage in the remote terminal to allowvoltage across the span between the source network element and a remoteline-powered network element to go back up.

Embodiments of the present invention further provide methods and systemsfor determining when the input voltage differs from a desired inputvoltage and when the difference exceeds a defined criterion. Thisenables a preemptory shut down of services to the network element beforedamage has occurred or a reduction in power usage in the networkelement. Other embodiments further provide an alarm signal when thevoltage difference exceeds the defined criterion. This allows thenetwork to shut down services to reduce power drawn by the line poweredelement when the input voltage falls and a indicator or flag has beenset.

Embodiments of the present invention provide a method for sensing theinput voltage at the network element sink, and detecting if it is closeto the critical point. If it is approaching this point, the powerconsumption of the network element sink is reduced to increase the inputvoltage and avoid power supply dropout and reboot.

Embodiments of the present invention provide a method for determining ifthe line power delivered on a span (communication line) is adequate tokeep the line powered network element running properly, with appropriatemargin headroom on the input voltage. If the input voltage is not highenough, an alarm may be generated and actions may be taken includingreduction in the quality of service to reduce power consumption of theline powered network element. Lower priority services may optionally bedisabled.

In one embodiment, the line powered network element utilizes a flybacktopology switching power supply. A diode on the secondary of the powersupply transformer rectifies the output current. A diode connected inthe opposite direction rectifies the reflected input voltage on thesecondary of the transformer, so that it may be compared to thresholdsfor action such as alarm generation, service reduction and powerreduction. The input voltage is filtered by capacitors and resistors toconvert the signal to a DC value that is proportional to the inputvoltage. This signal may now be measured by use of an analog to digitalconverter, a threshold comparator, or the like. It may also be used todirectly control power consumption reduction and service qualityreduction of the network element.

FIG. 1 is a diagram of one embodiment of a monitoring circuit coupled toa flyback power converter, shown generally at 100, according to theteachings of the present invention. System 100 includes one embodimentof a flyback power converter 102 and a monitoring circuit 110. In oneembodiment, flyback power converter 102 includes, switch Q1, first andsecond windings L1 and L2, a diode D1, capacitor C11, and an outputfilter 106. In this embodiment, output filter 106 includes an inductorL12 and a capacitor C12. It is understood, that output filter 106 is byexample and in other embodiments may comprise alternate components.Also, flyback power converter 102 is shown for illustration to includean output filter 106 and is not restricted to a flyback power converterwith an output filter.

In operation, flyback power converter 102 receives an input voltage Vin.During the on-time of switch Q1 a constant voltage V1, V1=(Vin−Vt)wherein Vt is the voltage drop across switch Q1, is applied across theprimary winding L1 of converter 102. When switch Q1 is turned off, V1drops to zero and the energy stored in the core causes the secondarywinding L2 to “fly back” and conduct current to the load. The voltage V2on the secondary winding L2 during the flyback time is determined by theturns ratio (the ratio of the number of turns in the secondary windingL2 of the transformer to the number of turns in the primary winding L1).Therefore the voltage V2 on the secondary winding L2 is proportional tothe input voltage Vin (assuming Vt is negligible).

In this embodiment, current does not flow simultaneously in bothwindings L1 and L2. Energy received from the input voltage Vin is storedon L1 and is transferred to L2 when switch Q1 is opened. In oneembodiment, flyback power converter 102 operates in a discontinuousmode. FIG. 2 a is a graphical illustration of the primary current Ip ofthe primary winding L1 of one embodiment of a flyback power converter,such as flyback power converter 102. FIG. 2 b is a graphicalillustration of the secondary current Is of the secondary winding L2 ofone embodiment of a flyback power converter, such as flyback powerconverter 102. In the discontinuous mode all the energy stored in theprimary winding L1 during the switch Q1 on-time is completely deliveredto the secondary winding L2 and to a load before the next cycle. Asshown in FIG. 2 b, there is also a dead time Tdt between the instant thesecondary current Is reaches zero and the start of the next cycle. Thedead time is when nothing in the transformer is energized. FIG. 2 c is agraphical illustration of a transformer primary voltage V1 for a flybackpower converter, such as flyback power converter 102.

When Q1 turns off, the current in the primary winding L1 forces thereversal of polarities on all windings. At the instant of turnoffideally all the energy from the primary winding L1 is transferred to thesecondary winding L2. When Q1 turns off and the energy is transferred toL2, D1 is forward biased and capacitor C11 is charged and an outputvoltage Vload is provided. Referring to FIG. 4, during this time D2 isreverse biased. When Q1 turns on, D1 becomes reverse biased, and D2conducts a small amount of current to charge capacitor C2 as shown inFIG. 4 to a value that is proportional to the input voltage Vin.Monitoring circuit 110 monitors the voltage V2 on the secondary windingL2 without drawing current from converter 102 and provides an outputvoltage Vout that is proportional to input voltage Vin.

FIG. 3 is one embodiment of a monitoring circuit shown generally at 310,according to the teachings of the present invention. In this embodiment,monitoring circuit 310 comprises a voltage sensing circuit 320 adaptedto selectively sense the voltage for a circuit or a power supply such asthe flyback power converter 102 of FIG. 1. In one embodiment, monitoringcircuit 310 senses the voltage V2 for the flyback power converter 102 ofFIG. 1. Voltage V2 is proportional to the input voltage Vin of flybackpower converter 102. V2 is indicative of the amount of voltage headroomavailable on the span used for line powering. In one embodiment, thecircuit is a network element as discussed below with respect to FIGS. 6,7, and 8.

FIG. 4 is one embodiment of a voltage sensing circuit, shown generallyat 420, according to the teachings of the present invention. Voltagesensing circuit 420 includes a diode D2 that permits a small amount ofcurrent to flow in voltage sensing circuit 420, therefore drawing littlecurrent from a respective circuit. Voltage sensing circuit 420 furtherincludes a capacitor C1 and a resistance-capacitance output filter 426comprised of resistor R2 and capacitor C2. An alternate output filtermay be substituted for filter 426. In some embodiments, output filter426 may not be included in the voltage sensing circuit 420. Outputvoltage Vout is representative of a measurement of the voltage at 450.

FIG. 5 is another embodiment of a monitoring circuit shown generally at510, according to the teachings of the present invention. Monitoringcircuit 510 includes a voltage sensing circuit 520 and a detectioncircuit 530. In one embodiment, voltage sensing circuit 520 is as foundin FIG. 4 above. In operation, monitoring circuit 510 senses the voltageat 550 and provides an output voltage Vout representative of the voltageat 550. Voltage sensing circuit 520 draws a small amount of current. Inone embodiment, detection circuit 530 includes a comparator circuit 540that compares Vout to a reference voltage Vref and when Vout differsfrom Vref by a defined criterion detection circuit 530 produces an alarmsignal 575. In one embodiment, alarm signal 575 provides an indicationthat the voltage provided to a line powered network element differs froma desired voltage. For example, in one embodiment the alarm signal 575indicates that the voltage to the line powered network element fellbelow a desired level of headroom.

FIG. 6 is a block diagram of one embodiment of a network 600 thatincludes at least one line-powered network element. Network 600 includesat least one network element 602 (referred to here as a “source networkelement”) that provides power to at least one other network element 604(referred to here as a “sink network element”) over a communicationmedium 606 (referred to here as a “power communication medium”). In theone embodiment, the source network element 602 is a central officeterminal located in a central office of a service provider and the sinknetwork element 604 is a remote terminal located in the outside plant,for example, in an environmentally hardened enclosure. In such anembodiment, both the central office terminal 602 and the remote terminal604 are included in an access network that is coupled to one or moreitems of customer located equipment (for example, a modem, wirelessaccess point, or telephone set) to a communications network such as theInternet or public switched telephone network (PSTN). The central officeterminal provides power to the remote terminal over at least onetwisted-pair telephone line. That is, in such an-embodiment, thetwisted-pair telephone line is the power communication medium.

The source network element 602 is coupled to a power source 608 in orderto obtain power that is used to power the source network element 602 andto provide power to the sink network element 604 over the powercommunication medium 606. In one embodiment, the power source 608includes a direct current (DC) and/or an alternating current (AC) powersource such as a battery and/or connection to a main power grid. Inother embodiments, other power sources are used.

The source network element 602 and the sink network element 604communicate with one another using some type of communication link. Forexample, in one embodiment, a central office terminal and a remoteterminal communicate over a DSL communication link provided between thecentral office terminal and the remote terminal. Examples of DSLcommunication links include a high-bit rate DSL (HDSL) link, high-bitrate digital subscriber line 2 (HDSL2) link, high-bit rate digitalsubscriber line 4 (HDSL4) link, or symmetric DSL link conforming to theInternational Telecommunication Union (ITU) standard G991.2 (a G.SHDSLLink). In other embodiments, other types of communication links areused.

In the embodiment shown in FIG. 6, the communication link is provided onthe same communication medium that is used to supply power from thesource network element 602 to the source network element 604. In otherembodiments, a separate communication medium is used to provide such acommunication link between the source network element 602 and the sinknetwork element 604.

Both the source network element 602 and the sink element 604 aretypically coupled to other network elements. For example, in oneembodiment, the source network element 602 is coupled to an upstreamnetwork element such as a switch and the sink network element 604 iscoupled to one or more downstream network elements such as various itemsof customer located equipment (for example, a modem, wireless accesspoint, or telephone set).

In one embodiment, source network element 604 includes a power supply618 that is coupled to the communication medium 606. The power supply618 extracts the power supplied on the communication medium 606 by thesource network element 602. The extracted power is used to power variouscomponents of the source network element 604. In one embodiment, powersupply 618 is a flyback power converter. In one embodiment, sink networkelement 604 further includes a monitoring circuit 610 coupled to thepower supply 618. In one embodiment, monitoring circuit 610 is asdescribed in the one or more embodiments described above with respect toFIGS. 1, 3, and 5.

FIG. 7 is a block diagram of one embodiment of a wireless network 700.The embodiment of a wireless network 700 shown in FIG. 7 includes acentral office power plug 702 that is coupled to a power source 704. Inone embodiment, central office power plug 702 is implemented using anembodiment of the central office terminal 800 described below. Anupstream G.SHDSL communication link 706 is provided to the centraloffice power plug 702 over an upstream communication medium (forexample, a twisted-pair telephone line). The upstream G.SHDSLcommunication link 706 couples the central office power plug 702 to aG.SHDSL line interface unit 708. The G.SHDSL line interface unit 708 iscoupled to an upstream network (not shown) such as the Internet. In onesuch embodiment, the G.SHDSL line interface unit 708 is inserted into asubscriber access multiplexer (not shown) in order to couple the G.SHDSLline interface unit 708 to the upstream network.

The wireless network 700 also includes a remote network element 710.Remote network element 710 is powered by a twisted-pair telephone line712 that is coupled between the central office power plug 702 and theremote network element 710. A downstream G.SHDSL communication link 714is provided over the twisted-pair telephone line 712. The central officepower plug 702 supplies power for the remote network element 710 on thetwisted-pair telephone line 712 in the same manner as described above inconnection with FIG. 6. The remote network element 710 includes a powersupply 718 that is coupled to the twisted-pair telephone line 712. Thepower supply 718 extracts the power supplied on the twisted-pairtelephone line 712 by the central office power plug 702. The extractedpower is used to power various components of the remote network element710.

In one embodiment, remote network element 710 further includes amonitoring circuit 709 coupled to the power supply 718. In oneembodiment, monitoring circuit 610 is as described in the one or moreembodiments described above with respect to FIGS. 1, 3, and 5.

The remote network element 710 also includes a G.SHDSL modem 720 thatmodulates and demodulates the G.SHDSL signals carried over thetwisted-pair telephone line 712. The modem 720 is coupled to a wirelessaccess point 722 over an Ethernet connection 724. The wireless accesspoint 722 transmits traffic to, and receives traffic from variouswireless devices (not shown) over a wireless link 726. Examples ofwireless devices include computers or personal digital assistants havingwireless transceivers. In one embodiment, the wireless access point 722is a wireless access point that supports the Institute for Electricaland Electronic Engineers (IEEE) 802.11b standard (also referred to as“WI-FI”).

The wireless network 700 also includes a wireless services manager 728that manages the wireless services provided over the wireless network700. For example, in one embodiment, wireless services manager 728manages authentication and other subscriber and service-relatedinformation using the Remote Authentication Dial-in User Service(RADIUS) protocol. In one embodiment, the wireless services manager 728is coupled to the G.SHDSL line interface unit 708 using a local areanetwork connection (for example, an Ethernet connection).

In operation, wireless traffic is received by the wireless access point722 from various wireless devices. The wireless traffic is transmittedto the central office power plug 702 by the G.SHDSL modem 720 over thetwisted-pair telephone line 712. A splitter (not shown in FIG. 7) splitsoff that portion of the signal used for providing the G.SHDSLcommunication link and provides it to a communications interface (notshown in FIG. 7) of the central office power plug 702 for appropriateprocessing. The communications interface transmits the traffic to theG.SHDSL line interface unit 708 over the upstream G.SHDSL communicationlink 706, where the traffic is processed and forwarded to the upstreamnetwork by the line interface unit 708. In the downstream direction,traffic is received by the G.SHDSL line interface unit 708 from theupstream network. The traffic is transmitted to the central office powerplug 702 over the upstream communication link 706. The traffic iscombined with power from a power supply (not shown in FIG. 7) of thecentral office power plug 702 by the splitter and the combined signal istransmitted on the twisted-pair telephone line 712. The signal isreceived by the G.SHDSL modem 720, which forwards the traffic to thewireless access point 722 for transmission to the wireless devices.

FIG. 8 is a block diagram of one embodiment of a central office terminal800. Embodiments of central office terminal 800 are suitable forproviding power to one or more remote terminals (or other networkelements) over one or more twisted-pair telephone lines (or othercommunication medium). The embodiment of a central office terminal 800shown in FIG. 8 includes communication interface 802 and a powerinterface 804. The communication interface 802 includes appropriatecomponents for providing the various telecommunications service providedby the central office terminal 800. For example, in the embodiment shownin FIG. 6, the communications interface 802 couples the central officeterminal 800 to at least one upstream G.SHDSL communication link and toat least one downstream G.SHDSL communication link (via a splitter 830described below). The downstream G.SHDSL communication links is providedover at least one twisted-pair telephone line 806. The twisted-pairtelephone line 806 is coupled, in one embodiment to one or more remoteterminals (not shown in FIG. 8) that are powered by the central officeterminal 800.

The power interface 804 includes a power supply 808 that is coupled to apower source 810. In general, the power supply 808 receives power fromthe power source 810 and conditions and supplies power on thetwisted-pair telephone lines 806 in order to power a remote terminalcoupled to the twisted-pair telephone line 806. In one such embodiment,the power supply 808 is implemented as a fly-back power supply. Thecentral office terminal 800 includes a splitter 830 that combines anoutput communication signal from the communications interface 802 and anoutput power signal from the power interface 804 and applies thecombined output signal to the twisted-pair telephone line 806. Thesplitter 830 also receives an input signal from the twisted-pairtelephone line 806 and splits off that portion of the received inputsignal used for providing the downstream communication link and providesit to the communications interface 802 for appropriate processing. Oneembodiment of a splitter 830 is described in a co-pending applicationentitled “SPLITTER”, Attorney Docket No. 100.592US01.

The power interface 804 also includes a controller 812 that controls theoperation of the power supply 808. In one such embodiment, controller812 is implemented in hardware (for example, using analog and/or digitalcircuits) and/or in software (for example, by programming a programmableprocessor with appropriate instructions to carry out the various controlfunctions described here). In other embodiments, the controller 812 isimplemented in other ways. Although the controller 812 is shown as beinga part of the power interface 804 in FIG. 8, in other embodiments thecontroller 812 is a part of a general controller or control circuitryfor the central office terminal 800. In other embodiments, the functionsperformed by the controller 812 are incorporated directly into controlcircuitry of the power supply 808.

In the embodiment shown in FIG. 8, a voltage signal 814 is providedbetween the controller 812 and the power supply 808. The voltage signal814 is used by the controller 812 to set a nominal voltage at which thepower supply 808 is to supply power on the twisted-pair telephone line806 in order to power a remote terminal coupled to the twisted-pairtelephone line 806. A power limit signal 816 is provided between thecontroller 812 and the power supply 808. The power limit signal 816 isused by the controller 812 to set a power limit for the power supply808. The power limit is a maximum power the power supply 808 is toprovide on the twisted-pair telephone line 806.

An overload signal 818 is provided by the power supply 808 to thecontroller 812. The overload signal 818 is used by the power supply 808to inform the controller 812 that the power supply 808 is currentlysupplying power with an output voltage that is below the nominal voltagespecified on the voltage signal 814. This is referred to here as an“overload condition” or that the power supply 808 is “out ofregulation.” For example, when a remote terminal coupled to thetwisted-pair telephone line 806 draws an amount of current that causesthe amount of power supplied by the power supply 808 to exceed the powerlimit specified by the power limit signal 816, the power supply 808drops the output voltage so that the total power supplied by the powersupply 808 does not exceed the power limit. When an overload conditionexists, the power supply 808 indicates that such an overload conditionexists on the overload signal 818.

In the embodiment shown in FIG. 8, various current measurement signalsare supplied by the power supply 808 to the controller 812. For example,a low current signal 822_is supplied by the power supply 808 to thecontroller 812 to indicate that the current currently supplied by thepower supply 808 is below some relatively low threshold current value. Ahigh current signal 820_is supplied by the power supply 808 tocontroller 812 to indicate that the current currently supplied by thepower supply 808 is above some relatively high current value. In otherembodiments, the amount of current currently supplied by the powersupply 808 is measured and provided to the controller 812.

A number of embodiments of the invention defined by the following claimshave been described. Nevertheless, it will be understood that variousmodifications to the described embodiments may be made without departingfrom the scope of the claimed invention. Accordingly, other embodimentsare within the scope of the following claims.

1. A voltage sensing circuit, comprising; a diode adapted to couple tothe secondary side of a flyback power converter; a first capacitoradapted to couple to the diode and filter voltage spikes that occurduring switch transitions on the primary; and an output filter adaptedto peak charge the output voltage, wherein the output voltage isproportional to an input voltage of the flyback power converter.
 2. Thecircuit of claim 1, wherein the output filter comprises aresistance-capacitance filter.
 3. A monitoring circuit, comprising: adiscontinuous flyback power converter; a diode coupled to a secondarywinding of the flyback power converter; a filter coupled to the diode; acapacitor coupled to the filter; and a voltage output coupled to thecapacitor, wherein the output is proportional to an input voltage on aprimary side of the flyback power converter.